The invention relates to a device for distributing vapor and liquid uniformly over the cross section of a vessel with two-phase concurrent downflow. The invention is suited for, but not limited to, the application of distributing hydrogen-rich treatgas and hydrocarbon liquid to the catalyst bed(s) in a hydroprocessing reactor, such as a hydrotreating or hydrocracking reactor.
A large number of distribution tray designs for two-phase concurrent downflow vessels have been described in literature and patents. The majority of these designs belong to one of the two categories given below:
Chimney Type of Distribution Trays:
These distributors consist of a horizontal tray plate provided with a plurality of chimneys extending up above the tray plate. The chimneys form flow channels for vapor flow across the tray plate. For the earliest distribution tray designs liquid openings for liquid flow where provided in the tray plate. For presently used distribution tray designs one or more lateral liquid opening(s) for liquid flow are provided in the side of the chimneys. These lateral liquid opening(s) may be at one or more elevations and may vary in size and shape. The total flow area of the liquid openings is selected to hold a certain liquid level on the tray, and the total cross sectional area of the vapor chimneys is normally selected to obtain a low pressure drop across the tray to ensure that the driving force for liquid flow through the liquid openings is mainly the static head of the liquid column above the liquid opening and not the pressure drop caused by vapor flow through the chimneys.
An example of a chimney type distribution tray is given in U.S. Pat. No. 4,788,040. A predistribution tray 56 is used above a final distribution tray 66. The final distribution tray 66 is a tray provided with liquid openings/perforations 84 for liquid flow and with capped chimneys 62 for vapor flow. The chimneys 62 are also provided with lateral liquid openings 90 for liquid flow. Distribution trays with liquid openings in or close to the tray plate have shown to be susceptible to fouling and plugging since particulate impurities tend to settle out on the tray and plug the liquid openings.
Another example of a chimney type distribution tray is given in U.S. Pat. No. 4,126,540. This distribution tray consists of a tray plate 33 provided with a plurality of chimneys 31. Each chimney is provided with one or more lateral liquid openings for liquid flow 34. All liquid openings are in the elevation H above the tray plate. A perforated plate 32 is located above the chimney tray. The perforated plate has perforations 30. No perforations are provided in the plate 32 directly above the chimneys 31. In this way direct liquid flow through the upper and open end of the chimneys is prevented. Another way of preventing direct liquid flow is by use of a chimney cap 24. This tray has improved resistance to fouling and plugging since the liquid openings are at a higher elevation and particulate impurities can therefore settle out on the tray without plugging the liquid openings. The drawback of chimney tray designs with liquid openings in one elevation only is a poor liquid flow rangeability. At low liquid flow rates the level will be at the liquid openings, and the liquid flow through each chimney becomes very sensitive to the variations in liquid depth which will always exist on the tray. At high liquid flow rates, liquid will overflow the lowest elevated chimneys and cause liquid maldistribution.
U.S. Pat. No. 5,484,578 describes a distribution system consisting of a predistribution tray 17 and a final distribution tray 18. The final distribution tray 18 is a chimney type of tray provided with a plurality of non-identical chimneys 33 and 34. The chimneys are provided with lateral liquid openings for liquid flow in one or more elevations. The chimneys 33 have one or more liquid openings at a lower elevation than the chimneys 34. In this way the liquid flow rangeability of the distribution tray is increased. The chimneys are provided with notches 38 to reduce the liquid maldistribution in case of liquid overflowing the chimneys.
Since the downward flow velocity inside the chimneys needs to be low, the exit flow pattern from the chimneys of the cited patent references is normally a low interaction flow pattern with liquid falling more or less vertically down from the drip edges of the chimneys. There is a limit on how close the chimneys can be spaced apart. With many chimneys on the tray with a close spacing, the liquid flow per chimney is low. Therefore the area of the liquid openings also needs to be reduced to still hold the desired liquid level on the tray. If the size of the liquid openings is less than about 15-30 mm2, then the liquid openings become susceptible to fouling and plugging. In other words there is a maximum chimney density which should not be exceeded if plugging of liquid openings shall be avoided. For typical chimney tray designs for hydroprocessing reactors, the maximum chimney density ranges from 50 to 100 chimneys per square meter to prevent plugging of the liquid openings. Due to the limited number of distribution points or chimneys, a certain liquid spread at the exit of each chimney is desirable to avoid a point-wise liquid flow below each chimney and no liquid flow in the areas between adjacent chimneys.
U.S. Pat. No. 5,403,561 describes the use of conical spray producing means 23 in the outlet of the chimneys 24 on a chimney distribution tray 22. The spray producing means may consist of metallic ribbon wound in the form of an inverted conical helix. The conical vapor/liquid spray will ensure a good local liquid distribution from each chimney outlet. It is intended that the spray of vapor and liquid, as it impinges on the top surface of the fixed bed 18, will overlap.
A second example on how a good local liquid distribution can be achieved in the outlet from each chimney is given in U.S. Pat. No. 6,613,219. A dispersive system 28, such as a perforated plate, is used below the chimneys to spread the liquid.
A third example on improved local liquid spread in the outlet from each chimney is given in International Publication No. WO 00/53307. A flow distributing element 10 consisting of radially aligned corrugated plates 22, 22a, 22b is inserted into the outlet 12 of a chimney 14 to produce a conical spray. The small size of the flow channels between the corrugated plates makes this design prone to plugging by solid impurities.
A drawback of all the referenced chimney trays is that the total chimney area needs to be large to achieve low flow rates and pressure drop in the chimney entrance. If the pressure drop for vapor entering the chimney is excessive, then this pressure drop will increase the pressure drop across the lateral liquid openings. The result is that the liquid flow rangeability of the distribution tray is reduced: In operating cases with low liquid flow and high vapor flow the liquid level will be even lower, and in operating cases with high liquid flow and low vapor flow the liquid level will be even higher. In addition the sensitivity of the liquid flow from each chimney to variations in liquid depth across the tray is significantly increased with increased pressure drop for vapor entrance to the chimney because there is a large change in liquid flow rate when the liquid level passes a liquid opening.
In commercial applications the chimneys may occupy as much as up to 30% of the total distribution tray area. Therefore a resistance to liquid flow across the tray plate exists, and liquid level gradients on the tray may occur. The liquid level gradients will result in liquid maldistribution. The chimney caps which prevent direct liquid flow into the chimney may occupy up to 50% of the total chimney tray area in commercial designs. A significant amount of the liquid which enters the distribution tray from above will therefore hit these caps. As a consequence the liquid which hits the caps needs to flow to the rim of the cap and fall down through the vapor entering the chimney. A significant amount of the liquid may therefore be withdrawn by the vapor into the chimney and does thus by-pass the lateral liquid openings, causing liquid maldistribution.
Bubble Cap Type of Distribution Trays:
These distributors have a completely different mode of operation than the chimney type of distribution trays. While the static liquid head is the driving force for liquid distribution on the chimney distribution trays, the driving force for liquid distribution on the bubble cap tray is the vapor flow. The bubble cap distributor consists of a horizontal tray plate. A plurality of distribution units or bubble caps is provided for vapor and liquid flow across the tray plate. Each bubble cap is an inverted U-shaped flow conduit consisting of upflow channel(s) and downflow channel(s). The lower part of each upflow channel is provided with one or more lateral vapor openings, typically vertical slots or inverted V-notches. Each downflow channel extends through the tray plate. The vapor passes through the lateral vapor openings in the lower part of each upflow channel and thereby generates a pressure drop from the vapor space above the tray to the inside of the upflow channel. Due to this pressure drop, liquid is lifted up into the upflow channel, mixed with the vapor and the two-phase mixture flows up through the upflow channel, over an internal weir and down through the downflow channel, and exits the distribution unit below the tray.
An example of a traditional bubble cap distribution tray is given in U.S. Pat. No. 3,218,249. The distribution tray consists of a tray plate 18 provided with a plurality of cylindrical downcomers 26 which serve as downflow channels. A cap 28 overlays each downcomer, and thus forms an annular upflow channel between the cap and downcomer. The cap is provided with slots, as indicated in FIG. 6. During operation, a liquid level will build up on the tray to a level intermediate the slots. Vapor will pass through the dry upper section of the slot. A pressure drop is therefore generated from outside the cap to inside the annular upflow channel. Due to this pressure drop, liquid is lifted from the liquid pool on the tray up into the annular upflow channel where it is mixed with the vapor. The two-phase stream first flows upward through the annular upflow channel, then the stream takes a 180° turn over the internal weir, consisting of the upper edge of the downcomer, and finally the two-phase stream flows downward through the downcomer and exits the bubble cap below the tray plate 18.
The downcomer and cap of the bubble cap distribution unit may have many different shapes and layouts, as illustrated in U.S. Pat. No. 5,942,162, where square and rectangular shapes of the cap and the downcomer are suggested in addition to the circular shape.
The bubble cap distributor has three major performance problems:                1. The liquid flow from each distribution unit is very sensitive to variations in liquid depth across the tray plate. This is especially true in applications with high vapor load.        2. In order to reduce the performance problem due to sensitivity to variations in liquid depth across the tray plate, the bubble cap distributors need to be designed with two-phase flow velocities inside the devices that are as low as possible. Since the vapor and liquid flow rates in the downflow vessel are fixed by other parameters, the available flow areas in the bubble cap, the upflow area, the flow area over the internal weir, and the downflow area need to be maximized to achieve the desired low two-phase flow velocities. Therefore the bubble caps occupy a large fraction of the total area of the distribution tray plate. In commercial designs up to about 50% of the tray area may be occupied by the bubble caps, and at the same time the liquid level is low, typically only 50-100 mm. As a result, the horizontal liquid flow velocity between adjacent bubble caps due to liquid flowing across the tray plate is high, and consequent large liquid level gradients on the tray may occur due to the frictional loss. The liquid level gradients will result in further liquid maldistribution. The resistance to liquid cross flow and thus the liquid level gradients are larger for bubble caps with a rectangular cross section than for bubble caps with a round cross section due to a larger frictional loss.        3. Pressure gradients exist in the vapor space above the distribution tray. The liquid flow from each bubble cap is sensitive to these pressure differences.        
U.S. Pat. No. 6,769,672 describes a bubble cap type distribution tray with significantly reduced sensitivity of the liquid flow from each bubble cap to variations in liquid depth across the tray plate. The improved bubble cap has two different types of upflow channels: upflow channels 16 with high vapor flow but low liquid flow, and upflow channels 15 with low vapor flow and high liquid flow. However the bubble caps still occupy a large fraction of the tray area, and liquid level gradients due to liquid flow across the tray are still a concern.
As mentioned, the two-phase flow velocities in the bubble caps must be kept as low as possible. This is in order to reduce the frictional pressure loss in the bubble cap. A low frictional pressure loss results in reduced sensitivity of liquid flow from each bubble cap towards the unavoidable variations in liquid depth across the tray. For this reason, attempts to improve the local distribution or spread of liquid at the outlet of each bubble cap by use of different inserts for liquid dispersion has failed. One example of such an attempt is given in U.S. Pat. No. 5,158,714. The inserts will represent a flow restriction and will increase the frictional pressure loss. As a consequence, bubble caps with inserts or other flow restrictions show significantly increased sensitivity of liquid flow from each bubble cap to variations in liquid depth across the tray, which again results in a poor tray-wide liquid distribution. A poor tray-wide liquid distribution can not be compensated by an improved local liquid spread at the bubble cap outlets.
Criteria for Proper Performance of a Distribution Tray
The following criteria must be fulfilled by a proper distribution tray:    A) There must be close to identical liquid flow rates from each of the distribution units on the distribution tray regardless of variations in liquid depth on the tray. The sensitivity to variations on liquid depth is quantified as the liquid maldistribution due to ½ inch “out-of-level” conditions, % Mal1/2 inch:
                              %          ⁢                      Mal                                          1                /                2                            ⁢                                                          ⁢              inch                                      =                                            2              ×                              abs                ⁡                                  [                                                            Q                      l                      low                                        -                                          Q                      l                      high                                                        ]                                                                                    Q                l                low                            +                              Q                l                high                                              ×          100          ⁢          %                                    (        1        )            
Where:                % Mal1/2 inch Is the percent liquid maldistribution due to ½ inch “out-of-level” conditions.        Q1high Is the liquid volumetric flow through one distribution unit elevated ¼ inch higher than average, m3/s.        Q1low Is the liquid volumetric flow through one distribution unit elevated ¼ inch lower than average, m3/s.        
Variation in liquid depth from distribution unit to distribution unit (out-of-level conditions) will always exist in commercial hydroprocessing units since:                1) The support ring, and thus the tray plate, is not in perfect level; see FIG. 10.        2) The tray plate and/or the tray support beams will deflect due to the load.        3) There are offsets in elevation of each individual distribution unit on the tray due to fabrication tolerances.        4) The liquid surface will be wavy due to the quite turbulent conditions above the distribution tray and due to liquid falling down from above.        5) There are often significant liquid level gradients on the tray due to liquid flowing across the tray. Often a radial flow from vessel centerline towards the vessel wall exists.        
A typical level difference in commercial reactors due to fabrication and installation tolerances (above points 1, 2 and 3) is about 0.5 inch.    B) There shall be many distribution units per m2 of tray area. With about 90 distribution units per m2 the liquid spread caused by a 200 mm thick layer of 1 inch diameter inert balls, located below the distribution tray, results in a uniform liquid distribution at the outlet of this inert particle layer which is typically the inlet to the active catalyst bed in a catalytic reactor. If means for improved local spread of the liquid exiting each distribution unit are used, then less than 90 distribution units per m2 may be acceptable. The coverage of the catalyst bed with distribution units shall be as uniform as possible. There shall not be areas near the reactor wall, thermowells or support structures, which are not covered by distribution units.    C) The distributor needs to be resistant to fouling, such as scales and particles. Such solid impurities will always be present in commercial applications.    D) Some liquid will pass with the vapor phase and will take the vapor path through the distribution unit. The liquid maldistribution caused by this effect shall be minimized.    E) Liquid entering the distribution tray is not evenly distributed. Some areas of the distribution tray receive large amounts of liquid, while other areas may not receive any liquid at all; see FIG. 11. Therefore it is important that liquid can flow across the tray from one area to another without creating excessive liquid level gradients.    F) Pressure gradients in the vapor space above the distribution tray exist since the kinetic energy of the high velocity exit streams from an inlet diffuser or an inter-bed mixer is converted into a pressure rise, so that the pressure near the reactor wall is typically higher than the pressure near the reactor center; see FIG. 12. A typical pressure difference in commercial reactors is about 50 Pa. There must be close to identical liquid flow rates from each of the distribution units on the distribution tray regardless of these pressure differences. The sensitivity to pressure differences is quantified as the liquid maldistribution due to a 50 Pa pressure difference in the vapor space above the distribution tray:
                              %          ⁢                      Mal                          50              ⁢                                                          ⁢              Pa                                      =                                            2              ×                              abs                ⁡                                  [                                                            Q                      l                      hp                                        -                                          Q                      l                      lp                                                        ]                                                                                    Q                l                hp                            +                              Q                l                lp                                              ×          100          ⁢          %                                    (        2        )            
Where:                % Mal50 Pa Is the percent liquid maldistribution due to 50 Pa pressure difference.        Q1hp Is the liquid volumetric flow through one distribution unit exposed to a 25 Pa higher than average inlet pressure in the vapor space, m3/s.        Q1lP Is the liquid volumetric flow through one distribution unit exposed to a 25 Pa lower than average inlet pressure in the vapor space, m3/s.        
Note that a pressure difference in the vapor space above the distribution tray causes a level difference as well. In the areas with low pressure, the level will rise, and in the areas with high pressure, the level will drop; see FIG. 12. % Mal50 Pa is the liquid maldistribution resulting from the combined pressure and level effect.